Are you planning to chop up an acoustic instrument? It takes a lot to make a good acoustic sound nice and just a little to screw it up. Fine guitars need to have the top tapered properly. Sometimes the top will be glued on tested, removed and redone until it's just right. Too much and it's fire wood. The quality of wood counts too. It seems like the standard template works well.
– ggcgDec 1 '18 at 21:58

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@ggcg No, I'd never try to take apart a good instrument lol. I'm thinking about making my own, but I want to maximize the volume.
– o.ma19Dec 3 '18 at 0:49

FWIW, world-class cello soloists play with the endpin mounted on a "sound box" which provides some additional volume (vs. the endpin touching the stage proper)
– Carl WitthoftDec 3 '18 at 16:11

2 Answers
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Actually, changing the geometry of the sound hole is very unlikely to change the overall volume.

As far as I can think, the only way to increase the volume of an already constructed string instrument is to put thicker strings on it. The additional mass should drive the bridge a bit harder. It probably won’t make a very big difference.

To make a louder sound box you need materials with a higher strength to weight ratio so they can be cut thinner and lighter so they will vibrate more easily. This is why solid Sitka spruce is arguably the best top material for a steel string guitar: it has a very high strength to weight ratio.

Another big factor is geometry and bracing. The more frequencies the sound box will resonate on, the louder it can be and the more even the tone. The choices of woods also can have an impact here. For the same woods and scale length, a larger sound box is usually louder. But if you don’t make sure there’s some high frequency resonates then the larger box might just be deeper and therefore could sound less loud.

Making sensitive (I.e., loud) stringed instruments with rich tone is a challenging art form, and pretty much all the parameters of that are decided before anything is glued together.

The holes in the body have almost nothing to do with the sound level. Remember, it's using the holes to project a sound wave (as, by comparison, a wind instrument's bell shape is designed for acoustic impedance matching (or mismatching) between the pressure wave and the free air outside the instrument).

You can never get more energy out of the instrument than is put in (duh), so the best you can do is to try to transfer as much of the string's vibrational energy into the soundbox as possible. Of course, if you were really successful, the string would be damped out in a few cycles -- also not desirable in general. As luser droog points out, the overall box design should avoid "hotspot" resonances which make a particular note (wavelength) project much more loudly than others.

So, as Todd W. pointed out, making quality string instruments is a very complicated art. Certainly you want the energy which is transferred via the bridge (mostly) to the body to be emitted via soundbox vibration -- which means you don't want the body to dampen the energy it has received.

I think in many important ways it is like a loudspeaker.
– luser droogJan 8 at 21:58

@luserdroog Maybe... I've opened up some medium-to-high-end speakers, and they often stuff the empty space in the box with insulation/sound-absorbing material. Unlike a musical instrument, the loudspeaker should not generate its own resonances and overtones.
– Carl WitthoftJan 9 at 14:35

I partly agree about the sound-hole. I think it functions more like a breathing hole to balance the air movement on either side of the top. But mostly the overtones (I have in mind "wolf tones" on violins) are tuned out in a vary similar manner to the design of a speaker cabinet. Particularly "resonator guitars" incorporate very similar design to speakers (which were invented at roughly the same time). I'm not sure exactly what my complaint is, but if you make any kind of edit then I can remove my downvote.
– luser droogJan 9 at 23:23